Central Locking Fault

BigTwig · Dec 3, 2018

Evening all. Odd fault today on the 2004 TD4, the remote central locking isn't unlocking the car. It locks on the fob, all doors, but unlock turns the interior light on and I can hear a small relay somewhere, but no doors unlock. I can use the key on the drivers and rear door, but the other 3 don't open. Even the interior unlock/lock button doesn't unlock the car, but does lock it - but not consistently! New battery in fob, and on remote locking, all the door actuators seem to fire. Getting 4 people and 2 dogs in the car through just the driver's and rear door is a PITA. Help please!!?!

saxavordian · Dec 3, 2018

Does your car key itself work all the locks. Does the tailgate drop with the fob a return with the key okay. To confirm after this disconnect the battery for half an hour and repeat the unlocking process. if this the same as before then diagnostic tool is needed.

BigTwig · Dec 4, 2018

> Does your car key itself work all the locks.

The key opens the drivers door and the tailgate.
> Does the tailgate drop with the fob a(nd) return with the key okay.

Yes, if I unlock the tailgate with the key, the window drops and tailgate opens when I pull the handle, as normal. Tailgate closes and locks OK.
> To confirm after this disconnect the battery for half an hour and repeat the unlocking process.

Given the above answers, do you think this is worth a shot? The fob is talking to the car, and the tailgate isn't confused.
> if this the same as before then diagnostic tool is needed.

Ah. Hope not. Thanks. I'll wait until the morning to try - if the alarm goes off, the neighbours will go ballistic

td4van · Dec 4, 2018

Tailgate key lock is only used to close the rear window - it doesn’t control the locking at all

I suspect that the door latches are knackered and need replacing.

Nodge68 · Dec 4, 2018

Does pressing and holding the unlock button cause the tail door window to drop? If so then the latches need changing as suggested. Or if the fault suddenly affected all the door, then it could be the relay in the CCU.

BigTwig · Dec 4, 2018

Ah, press and hold DOES cause the rear window to drop. I'll investigate the latches then.. Thanks!

BigTwig · Dec 4, 2018

Nodge68 said:
the latches need changing

I can't believe ALL the latches would fail at once. Same symptoms if just one of them fails? Any way to find out easily? I guess the driver's side may be a "master" and therefore more likely to be the one that's failed? Thanks.

Nodge68 · Dec 4, 2018

BigTwig said:
I can't believe ALL the latches would fail at once.

It's unlikely that all latches would fail at the same time.

BigTwig said:
I guess the driver's side may be a "master" and therefore more likely to be the one that's failed?

The driver's door is the same as the other doors, so not a master at all.

The CCU controls the locking motors to all doors. So that's the place to start looking. The locking motor wire colours are Orange, Pink and Brown/Pink.
The CCU controls 2 different locking motors in each lock assembly, swapping the polarity as needed to lock or unlock the doors, using power supplied on those 3 wires.

td4van · Dec 4, 2018

If it locks off the remote but won’t unlock off the remote then the wiring and CCU are fine in my opinion.

Sounds to me like the latches have failed and maybe over time and just been noticed as more are not working.

But before spending on new latches doesn’t harm in checking wiring to latches.

BigTwig · Dec 6, 2018

Just to wrap this up, I fixed the CCU late last night. Relays had erroded contacts. Two cleaned up OK using fine emery paper, the 3rd (dual relay) was too pitted so removed and replaced with two separate gash 12V relays I savlaged off other electronic kit. Now all working. Thanks for your help lads

BigTwig · Dec 6, 2018

Thought I'd add some pics. You can see the dirty contacts and where I'd removed one of the (dual) relays. I wouldn't recommend breaking into sealed relays like I did here - very easily damaged - but it was very late and my patience had already gone! I did tape them up after cleaning. A better fix would be to desolder them all and replace like for like, they're available on Ebay. The outer two are NEC ep2-3l1st (twin changeover relays in one package) and the centre one is an ep2-3l1s (an odd, ganged twin relay).
On the relays I installed they were dual contacts each, which I "parallelled" for double the current handling capacity as they were slightly underrated otherwise. I used flying leads and cut an extra exit from the CCU box so they can live outside the box. If I have to do this again, I'll buy new relays and do it properly.
Just glad the latches and door locks were OK - I hate door card removal jobs!
Oh, still need to replace one bolt that holds the metal fusebox surround to the steering column - it's a PITA to reach.
One last comment, I suspect the relay contacts are erroded not just because of the high current draw of the motors, but because of the back EMF from the motor coils when they're powered off.

Nodge68 · Dec 6, 2018

Well done on the repair. My hunch that it was a relay was correct.

Central Locking Fault

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